The present invention relates to a zoom lens for forming an image on an imaging element such as a CCD sensor and a CMOS sensor.
In these years, there are studies on mounting a zoom lens on a small device such as a cellular phone, a portable information terminal, and an internet camera as well as a digital still camera for another additional value, and devices actually equipped with zoom lenses became available. A zoom lens is an optical system, in which a part of lenses or lens groups that compose a lens system moves along an optical axis, whereby it is possible to continuously change imaging magnification. Mounting a zoom lens, it is possible to increase and/or decrease an image of an object to any sizes upon taking the image.
In case of mounting such zoom lens onto a small-sized device, the whole length of the zoom lens is preferably as short as possible. In case of a zoom lens, however, since it is necessary to move at least two of lens groups that compose the zoom lens upon changing magnification and focusing, it is required to secure space within the zoom lens to be able to move those lens groups therein, which makes difficult to attain miniaturization of the zoom lens.
On the other hand, the number of pixels to take an image as electrical signals has increased each year, and therefore a zoom lens is also required to exhibit high performances such as satisfactory aberration correction performance and compatibility to high resolution.
Patent Reference has disclosed a zoom lens, which includes a first lens group that is composed of a lens having negative refractive power; a second lens group that is composed of two lenses, a positive and a negative lenses, so as to have negative refractive power as a whole; a third lens group having positive refractive power; and a fourth lens group having positive refractive power. According to this zoom lens, restraining a composite focal length of the first lens group and the second lens group at a wide-angle end within certain length, it is possible to attain relatively satisfactory miniaturization in spite of its highly variable magnification, which is as high as three times.    Patent Reference: Japanese Patent Publication No. 2001-343588
The zoom lens described in Patent Reference does not fully satisfy the demands for high performances and miniaturization, although it is possible to relatively satisfactorily correct aberrations with a small number of lenses.
Here, such demands for high performances and miniaturization are not demanded only in small-sized devices such as cellular phones. Even in devices such as digital still cameras, there is a demand for changing magnification of an image, especially optically changing of magnification with less image deterioration, whereas there is also a demand for a smaller thickness to enhance portability.
In view of the above-described problems, an object of the invention is to provide a small-sized zoom lens with high performances that can provide satisfactory high image quality.